Is it possible to put a class (not an instance) in a variable or array element, and to use this to call a class method?

I'm defining a hierarchy of classes and it would be convenient if it were possible to register the fact that a class had a particular subclass, such that the information was available before instantiation.

I'm assuming that the code that inspects the array has imported all units so is aware of the relevant class declarations, but I am still having problems with calling class methods and/or casting to a class (as distinct from an instance).

In our previous episode, Mark Morgan Lloyd said:

Afaik this route you can only call static methods of the basetype of the class-type.

So if it is

     A= class
          end;
     b=class(a)
          end;
     c=class(b);
     TClasstype  = class of  B;
var x : TClasstype;
begin
  x:=C;
  x.staticmethod

then staticmethod must be visible in B. It doesn't matter which descendant of B you assign to X.

This is logical, since static methods are entirely compiletime, and the compiler can't know what type will be runtime assigned.

Thanks Marco, I was beginning to suspect that I was hitting limitations and that what I was trying to do might be ill-advised.

I'm finding virtual class methods useful when I'm able to refer directly to a class, rather than going via a reference to a class.

I'm doing something wrong- it works.

I've got a sequence of classes, each with a Parent class procedure and Children() class function which reference a global array associated with each class (hardcoded name). Each class also has an Oid() function which appends a hardcoded number to whatever it's inherited.

If a test program does this

WriteLn(OidToStr(TIso.Oid));
WriteLn(OidToStr(TIso.Children[0].Oid));
WriteLn(OidToStr(TIso.Children[0].Children[0].Oid));
WriteLn(OidToStr(TIso.Children[0].Children[0].Children[0].Oid));

I get this

.1
.1.3
.1.3.6
.1.3.6.1

i.e. each reference to Children() is identifying the correct method, despite the fact that this is spread over more than one file.

A "class reference", which is the official name of a "class of xxx" type, is basically a fancy monicker for the VMT of a class. So it's normal that calling virtual class methods via class references works as you'd expect.

Jonas

It was the "class of" bit that I was initially missing- and I think I used the term "reference" in my initial question because I was gradually getting my head around the various issues.

In actual fact I was looking at the Forward type declarations section in the Reference guide yesterday and trying to work out exactly what "only possible with pointer types and classes" meant- pointers are adequately covered but I wonder whether an example of the latter form would be appropriate, even though it precedes the chapter on classes?

Are you looking for something like this?:
http://code.google.com/p/luipack/source/browse/trunk/luicontrols/wizardcontrols.pas#266

Luiz

No, I'm doing something simpler- setting up a sequence of classes to mimic the definition of a node in an SNMP MIB file. I've got to the point where I can query (the Pascal transcription of) a MIB file to find out what nodes it implements and can work backwards and forwards to list the OIDs associated with a node and its ancestors-descendants- all without instantiating anything.